The prior art includes detectors for selectively detecting explosive vapors such as dynamite and tri-nitro-toluene (TNT) in the atmosphere. One effective prior art detector is disclosed in U.S. Pat. No. 4,116,042, which issued to Anthony Jenkins on Sept. 26, 1978 and shows a system for continuously detecting explosive vapors which gave a rapid response in less than one second of sampling the vapor. As shown schematically in FIG. 1, air was drawn into this prior art detector through a heated nozzle 21 by the action of a small suction pump 30. The hot air flow was caused to impinge onto a heated elastomeric membrane 22 made of di-methyl silicone rubber. The membrane 22 was chosen because of a favorable property which allows low volatility vapors to diffuse through the membrane while blocking the transport of unwanted gases such as oxygen. Explosive vapors were transmitted in this prior art detector with an efficiency of about 2%.
Explosive vapors which were transmitted through the membrane 22 in the prior art apparatus of U.S. Pat. No. 4,116,042 were carried on an inert carrier gas stream and split into two flow paths. The two flows were passed down two, short open tubular chromatographic capillary columns 25 and 26 toward detectors 27 and 28 respectively. The capillary column 26 was coated with a polar chromatographic liquid, while the capillary column 25 either remained uncoated or was coated with non-polar material. The flows in the prior art device of U.S. Pat. No. 4,116,042 were arranged such that common interfering compounds, such as chlorinated solvents, travelled down the two columns 25,26 and arrived at detectors 28 through the polar column 26 a few milli-seconds before arriving at the detector 27 down the non-polar column 26. Explosive vapors are mostly very polar and are absorbed strongly down the polar column where they are delayed. The explosive vapor peak arrived at the detector through the non-polar column 25 before the polar detector 28.
The signals of the two detectors 27 and 28 in the prior art detector were processed, and the difference between the two signals was computed in an analogue summing amplifier. The difference signals generated in the prior art detector for explosive and non-explosive vapors are shown in FIGS. 2a and 2b. It can be seen that the difference signal for explosives responds in the opposite direction to non-explosives. A logic gating circuit was arranged in the prior art detector to allow the signals to be presented at an output device such as an audible alarm or visual readout when the difference signal went negative while the signal strength was increasing.
The prior art system shown in FIG. 1 and described in greater detail in U.S. Pat. No. 4,116,042 provides excellent sensitivity and selectivity to most explosives. Unfortunately, small amounts of explosive vapor in the presence of halogenated materials cannot be detected in this prior art device because the difference signal for halogens overpowers the difference signal for explosives. A further limitation of the prior art device is that although the electron capture detector itself may be sensitive to one or two parts of explosive vapors in one trillion (10.sup.12) parts of pure carrier gas, the concentration gradient developed across the membrane reduces this detection limit by a factor of 10 or more. Furthermore, the impurity "noise" in the atmosphere causes fluctuation in the concentration of responding vapors reaching the detector in the prior art device. This increases noise levels also by a factor of ten. The net effect is that the limit of detection of the prior art device is one part of explosive vapor in 10.sup.10 parts of air. This is insufficient to detect the vapors from the "plastic" explosives largely comprised of cyclotrimethylenetrinitramine (otherwise known as RDX), and pentaerythritol tetranitrate (PETN). A sensitivity to one part of RDX in 10.sup.12 parts of air is required to provide a means of detection for the plastic explosives.
In other known devices, air samples are collected in a trap before being desorbed into a sensitive detector such as an electron capture detector (ECD). Unfortunately, several materials commonly present in the atmosphere respond on the ECD and have to be separated from the explosive vapor in a chromatograph separation process. If the vapors from dynamite, TNT and plastic explosives are sought by this technique, then several minutes are required to separate these compounds from the other, mostly volatile, interfering compounds in the atmosphere. One further major disadvantage with this technique is that sampling valves are normally deployed to control the sampling and desorption processes. These present large surface areas for irreversible adsorption of the very low volatility explosive vapors. Most, if not all, of the vapors from RDX and PETN are lost in the system and are never detected. No such detector has been successfully developed to detect these vapors in the quantities necessary to achieve detection of terrorist bombs made from plastic explosives.